1. Field of the Invention
The present invention relates to lithium-ion accumulators which include an electrolyte containing at least one electrolyte salt in a solvent and at least one additive. Furthermore, the present invention also relates to a method for preventing the decomposition of active electrode material.
2. Description of Related Art
In many sectors, e.g., in mobile phones, camcorders and laptop computers, but lately also in electric vehicles and electro-hybrid vehicles, lithium-ion accumulators are used as rechargeable electrochemical voltage source. The multitude of application fields leads to increasing demand for improved, highly reliable lithium-ion accumulators which have a high energy density and sufficiently long service life representable by the number of charge and discharge cycles. Known lithium-ion accumulators currently have a relatively short service life of approximately three to five years. An exception are accumulators for special application fields, e.g., for space technology.
Conventional lithium-ion accumulators include a cathode (positive electrode), an anode (negative electrode), a separator situated in-between, which separates the oppositely charged electrodes from each other, and an electrolyte, which establishes the electric connection between the positive and negative electrodes spaced apart from each other, and which may be employed in the form of a liquid electrolyte or a gel electrolyte.
Liquid electrolytes used in lithium-ion accumulators typically include a lithium salt, which is solubilized in one or a plurality of solvents, typically nonaqueous, aprotic organic solvents. When using an accumulator, lithium ions (Li+) are transported from the negative electrode (anode) to the positive electrode (cathode) through the electrolyte during the discharge, and electric energy is released in the process. The flow of the lithium ions is reversed during charging, the ions being transferred from the cathode through the electrolyte, back to the anode. In general, the anode and cathode of a lithium-ion accumulator include an active anode or cathode material that is suitable for the method of functioning of the accumulator, which material is electrochemically active and suitable for absorbing lithium ions, as well as a binding agent and a conductive material.
Already known are accumulators of the type referred to as “rocking chair”, in which a carbon material, e.g., graphite, is used as anode material, which during the charge operation is capable of intercalating (incorporating) lithium ions at the intercalation locations of its lattice planes formed by carbon atoms in the shape of six-membered rings. Typically, a lithium intercalation material such as LiCoO2, LiNiO2 or LiMn2O4, which is capable of deintercalation (removal) of the lithium ions from their intercalation locations, is used as active cathode material, so that lithium ions move back and forth between the interstitial electrodes during the charge/discharge cycles.
Typical electrolytes of such lithium-ion accumulators include one or more lithium-containing electrolyte salts in a solvent, i.e., a lithium cation having an anion. Examples of such electrolyte salts are LiClO4, LiI, LiSCN, LiBF4, LiAsF6, LiCF3SO3, LiPF6 and the like.
In such accumulators it is important that impurities, which can affect the accumulator capacity, are removed to the greatest extent possible. For example, the reaction of the lithium, which moves between the electrodes, with impurities causes a passivation layer to form on the anode. This consumption of the lithium reduces the capacity of the accumulator. Another possible reason for the capacity drop of a lithium-ion accumulator can be traced to an undesired reaction of the water that is present, with the components of the electrolyte included in the accumulator. For example, published U.S. patent application document 2003/0190530 describes the reciprocal effect of water with LiPF6, which is known as a typical electrolyte salt. Due to the resulting reciprocal effect, the internal resistance of the accumulator rises as a result of the decrease in the quantity of conductive components, gas and oxidizing substances being produced as well.
Published European patent document EP 0 947 027 describes a reaction of the water present in the accumulator with the electrolyte or with the lithium salt solubilized in a solvent. The water reacts with the solubilized lithium salt while forming a hydrogenous acid, which subsequently is able to cause an acid oxidation (acid attack) of the active cathode material, in particular when the cathode material is a lithium metal oxide. This acid oxidation leads to a decomposition of the active cathode material, during which water is produced once again, which is then able to react anew with the lithium salt of the electrolyte. In so doing, the acid environment is increased further. A chain reaction therefore results, which causes a cumulative corrosion of the active cathode material. The produced decomposition reaction is not related to the quantity of contaminating water initially present in the accumulator, but theoretically continues for as long as reagents can be produced from the active cathode material. Published European patent document EP 0 947 027 describes a composition of a lithium-ion cell and a method for preventing the decomposition of one or more components contained therein. By incorporating an additive, e.g., an alkaline compound, which represents an electron donor, a portion of the produced hydrogenous acid is neutralized by reacting with this donor. The alkaline compound may be added to the active material of the cathode, or it may be added to the electrolyte solution as additive, the additive being mixable with the electrolyte solution or being soluble therein. Such alkaline compounds are, among others, carbonates, metal oxides such as aluminates, hydroxides, amines, organic bases and silicates on lithium basis. The fact that the share of active cathode material is reduced by the incorporation of the additive, which thereby reduces the capacitance of the accumulator, has been shown to be disadvantageous. In the same way, interference with transport processes to and inside the electrodes may occur. Since these additives become effective as interceptors only when the electrode is attacked, important cover layers as well as the electrode surface may be damaged.